Disclaimer. Don't rely on these old notes in lieu of reading the literature, but they can jog your memory. As a grad student long ago, my peers and I collaborated to write and exchange summaries of political science research. I posted them to a wiki-style website. "Wikisum" is now dead but archived here. I cannot vouch for these notes' accuracy, nor can I even say who wrote them. If you have more recent summaries to add to this collection, send them my way I guess. Sorry for the ads; they cover the costs of keeping this online.
Denny, Kevin; Doyle, Orla. 2009. Does Voting History Matter? Analysing Persistence in Turnout. American Journal of Political Science 53 (January): 17-35.
Denny and Doyle have a straightforward point in this article: Yes, voting is habit-forming, but to a lesser extent than reported previously. In a widely discussed article, Gerber, Green, and Shachar (2003) reported that voting in one election raises the probability of voting in subsequent elections by 47%. Denny and Doyle argue that the correct figure is closer to 13%. The difference, they claim, arises from methodological problems in the Gerber et al. article.
Given that this dispute revolves mostly around methodological (not theoretical) differences, one wonders why this article did not appear as a reply to Gerber et al, followed by a response from the original authors. As my critique at the end of this review will make clear, there are many holes that Gerber et al could poke in Denny and Doyle's approach.
Those familiar with Gerber et al's 2003 article may be surprised to see methodological arguments leveled against it. After all, Gerber et al used sound experimental methods to neutralize methdological concerns. They selected a sample of voters who had not turned out in recent elections; they randomly assigned some of those non-voters to receive a mobilization treatment in 1998; of those who actually turned out in 1998 as a result of this mobilization effort, they then looked to see how many turned out again in 1999. At each stage, turnout information for each voter was gathered from public records--not from self-reported polling data.
The problem, Denny and Doyle claim, is not so much with the research design as with the statistical analysis. Gerber et al use instrumental variables in a lagged binary choice model, a procedure that does not yield consistent estimators. As a result, Denny and Doyle write, it appears that Gerber et al severely overestimated the effect that voting in previous elections has on voting in subsequent elections--that is, Gerber et al overestimated the extent to which voting is habit forming.
Denny and Doyle seek to estimate the strength of "habit" more accurately by using Britain's National Child Development Study (NCDS), a six-stage panel that tracked children born in March of 1958 into the late 1990s. They use a large number of time-variant and time-invariant variables from this study to predict turnout in 1979, this cohort's first opportunity to vote. For later elections, they use the same background variables, but insert a lag for whether the respondent had voted in previous elections. This procedure produces the estimate that voting in previous elections raises the probability of voting in subsequent elections by 13%.
Punchline: Yes, voting is habit-forming, but not as much as Gerber et al thought.
I'm surprised that AJPS did not provide Gerber et al the opportunity to reply to this article. I hope to see a reply from them in a future issue. Even if they accept Denny and Doyle's econometric criticisms as valid (which they appear to be), I suspect that Gerber et al would find plenty of flaws in Denny and Doyle's approach.
For example, I find it surprising that Denny and Doyle think their statistical control variables can remove individual-level background conditions (fixed effects) as well as Gerber et al's randomization can. As Denny and Doyle acknowledge near the beginning of their article, the greatest empirical obstacle to identifying an effect of habit is that you must first control for anything that might influence an individual voter's propensity to vote. If you omit a single variable that influences propensity to vote, then that variable's effects will be swallowed up in the lagged turnout measure--inflating the estimated effect of habit.
Gerber et al use randomized experimentation to completely remove concerns about omitted variables; Denny and Doyle by contrast, use a (large) vector of control variables taken from the NCDS. They make a valiant effort to argue that these variables (and their sophisticated methods) can account for this problem, but let's face it--no matter how many control variables you use, control variables can never improve on randomized experimentation.
More broadly, I wonder whether this study is even comparable to Gerber et al's. Gerber et al studied American elections, and argued that habit raised your probability of turning out by 47%; Denny and Doyle study British elections and find that habit raises your probability of turning out by 13%. But do we really expect habit to have the same effect in such different contexts? Elections are far more frequent in the United States than in Britain--so much so that some worry about "voter fatigue" hurting American turnout. This difference alone implies two things relevant to habit formation. First: Since American elections are more frequent, voters have more opportunity to develop a strong habit. Second: Since American elections happen in more rapid succession, there is less time for the habit to "wear off" before the next election occurs. (Recall that Gerber et all compared turnout in 1998 to 1999--only one year apart; is it any surprise that this would have a stronger effect than comparing British turnout in 1979, 1987, and 1997?)
In sum: An interesting article, but I'd like to see more. I guess that's how I end most of my reviews, though.
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